Virtual flood-electron source



Feb. 18, 1958 H. M. SMITH I 2524259 VIRTUAL FLOOD-ELECTRON SOURCE Filed June 6, 1955 zzazem @w-zz [k /74x. J {gum M Jam/4 Mil-lb AV 2,824,259 Patented Feb. 18, 1958 VIRTUAL FLOOD-ELECTRON SOURCE Application June 6, 1955, Serial No. 513,221 7 Claims. (Cl. 315-12) This invention relates to storage type cathode-ray tubes and more particularly to an electron source for flooding the storage electrode in a storage tube with electrons having substantially the same approach velocities normal to the plane of the storage electrode and phosphor display screen.

An electron source called a flood gun is generally provided in a cathode-ray type storage tube to reproduce information stored on a foraminated storage electrode. The flood electrons are directed over the entire surface of the storage electrode. In order to cause electrons to impinge upon the storage electrode with substantially uniform normal velocities, it is necessary to make the usual cathode source of the flood electrons relatively small to position the source as far from the storage target as is practically possible, and to place the source as near the symmetrical axis of the tube as possible. It is usually impracticable to put the flood gun exactly along the axis of the tube, because the focusing and deflecting means for the high voltage recording, or writing, beam is directed along that axis; and it is desirable that neither the beam nor its control means interfere with the flood gun. In the design of a flood gun for a cathode-ray type storage tube all the above recited design considerations are usually necessary but all are undesirable. One disadvantage of a small electron source is that the available current is small. When the flood gun is spaced a considerable distance from the storage tube target electrode, positive ion generation prevents a good reproduction of the charge pattern. Also for a half-tone storage tube, positive ion current limits the usefulstorage time of the storage electrode. Furthermore, actual symmetry of the flood guncan never be achieved unless it is placed on the symmetrical axis of the tube. Recent developments in direct-viewing storage tubes have made it imperative that variationof the electron velocities be minimized.

The flood electrons transmitted through the openings in thestorage electrode are utilized in direct-viewing storage tubes'to illuminate a phosphor viewing screen. This transmitted flood current in a tube then rises from essentially zero to that required for full brilliance when voltage of the storage electrode surface varies as little as three volts. This voltage control range has recently been expanded to as large as fifteen or twenty volts; however, the production'of flood electrons having uniform normal velocities when approaching the storage electrode is' still extremely desirable. This is particularly true when reasonable accuracy is required in producing intermediate illumination for half-tone operation. For a storage tube utilizing intermediate illumination, reference may be made to .a copending application, Serial No. 459,403, filed September 30, 1954, by E. E. Herman and G. F. Smith, entitled Direct-Viewing Half-Tone Storage Device, which is now U. S. Patent No. 2,790,929, granted April 30, 1957.

It is therefore an object of the invention to provide an improved flood electron source for a cathode-ray type storage tube.

It is another object of the invention to provide a storage tube electron source flooding the storage electrode with electrons having substantially the same normal velocities when they approach the storage electrode.

In accordance with the invention, an annular cathode is positioned about the symmetrical axis of a storage tube to provide a symmetrical electron flood having a uniform normal velocity distribution. A metallic grid is positioned on each side of the annular cathode for forming an electron cloud or a virtual flood cathode between the grids. An annular focusing electrode is then positioned externally about the cathode to direct the electrons emitted from the annular cathode into the space between the grids.

The novel features whichare believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which two embodiments of the invention are illustrated by way of example. It istto be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. l is a sectional view of a direct-viewing storage tube incorporating an embodiment of the virtual electron source of the present invention; and

Fig. 2 is an enlarged broken-away sectional view of another embodiment of the electron source of the present invention.

Referring to Fig. 1, there is shown a storage tube 10 having an evacuated envelope l2 and provided with a recording and deflection voltage generator 14.

Envelope 12, which may be made of glass, has a neck portion 20 enclosing a writing beam electron gun 22 and an enlarged portion 24'having a viewing end 26. The viewing end 26 may be made of any suitable clear glass and coated internally of the envelope 12 with a luminescent phosphor material 28.

Writing beam gun 22 comprises a cathode 30 which is provided with a filament 32, a current control grid electrode 34 and an accelerating anode 3S. Cathode 30 is maintained at a few thousand volts negative with respect to ground. Accelerating anode 38 is then maintained at ground potential.

Painted onto, the internal surface of the envelope 12 is a conductive coating 46 which may be made of a commercial preparation known as Aquadag. Conductive coating 46 is also maintained at ground potential. Conventional deflection yoke is shown disposed about neck porion 2% and coupled to deflection voltage generator 14.

A flood electron source 48, constructed in accordance with the present invention to produce a flood electron stream. 49, is shown in the enlarged portion 24 of the envelope 12. Electron source 48, which is employed to produce flood electrons, comprises a single annular wire filament 5'0 which is also shown in Fig. 2, having a hollow toroidal metallic cathode sleeve 52 disposed thereabout. Annular filament 50 is connected across a source of potential 56 and cathode sleeve 52 is maintained at ground potential.

A toroidal focusing electrode 58 is disposed externally about partially enclosing cathode sleeve 52. The focusing, electrode 58 is also mechanically connected to back and front metallic grids 57 and 59, respectively, Which are disposed on both sides of the cathode sleeve 52. Control electrode 58 and back and front grids 57 and 59 are maintained at the same potential as cathode sleeve 52.

Filament 50 could be used without the cathode sleeve 52 to provide 21 directly heated cathode; however, the

source 48. Hence, an indirectly heated cathode, viz

cathode sleeve 52", is preferably employed as a unipoten tialosource of electrons. An electron emissive layer 66 is deposited on the cathode sleeve 52'. This is more 7 clearly, indicated in Fig,- 2.

Focusing, electrode 58' need not have the exact shape illustrated,.but may be constructed simply as a hollow cylindricalcup or may have any other convenient shape. The -potential of control electrode 58 may be kept somewhat below or equal to thepotential of cathode sleeve 52' in order to-cause electrons emitted'f rom cathode sleeve 52'to be initially confi'nedin the space betweenfront and backgrids 57' and 59;

Between electron source 48 andthe'luminescent material- 28 on the viewing end' 26 of the envelope 12,. a screen gride1ectrode'72 is disposed adjacent storage electrode T4. Electrode 74, which is em'ployedasthe storage device, may comprise an electroformed' mesh having patches of a dielectric storage materialdisposed' over its left surface in the drawing.

Screen grid 72 is maintained at a potential a few hundied volts positive with respect to ground by a connectionto' the positive terminal of potential source 71.

- The flood electron source 48' is easily mountable within thejenla'rgedp'ortion 24 of an'envelope 12,,as shown. The fact'that the flood gun 48may be mounted there is advantageous for two reasons, viz., the unusual proximity ofth'e' fioo'd gun 48 to storage electrode 74 reduces positive ion generation, and the writing beam developed by gun 22 may be usefully focused and deflected without I interfering with the operation of the flood electron source 48.

In the reproducing operation of the storage tube the'flo'od beam 49'isdirected toward-the storage electrode,

74 and those electrons which penetrate the storage electrode illuminate the screen 28' according to the electrostatic chargepa'tternrecorded on the storage electrode. The uniform velocity spread over the fl'ood beam 49 then permits half-tone operation wherein the brilliance of an elemental area on the viewing screen 28 is caused to be proportional to the amount of electric charge stored' on the storage electrode 74.

An alternative embodiment of the electron source of the present invention is shown in Fig. 2. An accelerating field is employed in an electron source 61. The filament 50' is also shown in Fig. 2 with cathode sleeve 52, electron emissive layer 66, back and front grids 57, 59, potential source 56,. and the fl'oodlbeam 49. A focusing electrode 55, different in construction than that of focusing electrode 58, is disposed about cathode sleeve 52 and electron emissive layer 66' so as to both focus the electrons emitted at the electron emissive layer 66 into the space between back and front grids 57, 59 and to control the current in the flood stream 49. For this reason an arcuate aperture 120 in the toroidal shaped focusing electrode 55 is maintainedata potential a few volts negative with respect to that. of the cathode sleeve 52. The front and back grids 57,- 59" are maintained at a potential generally less than 100 volts positive with respect to that of cathode sleeve 52 by means of a potential source 51.

In the operation of the electron source 61 electrons emltted at the electron emissive layer 66 and thus directed through the aperture 120 in focusing electrode 55 and accelerated into the space between back andfront grids lope; two metallic-grids disposed one on each sideof said annular cathode, means for directing electrons emitted from said annular cathode radially inward into the space between said metallic grids, and an attracting electrode disposed between one of said metallic grids and the storage electrode for: accelerating the electrons directed intothespacebetween said gridstowardthev storage electrod'e'and for collecting secondary electrons liberated from therstoragegelectrodea 2; In a; cathode-ray; type: storages tube: including an evacuated envelope having a storage electrode disposed at one end, and awriting beam gun adapted toscan the storage electrode: an annular cathode disposed transversely to and symmetricallyaboutlhe longitudinal axis of the envelope; two metallic grids disposed one on each side of said cathode; means for directing electrons emitted from said cathode radially inward into the space between said" metallic grids; and means for directing said emitted electrons through the, interstices of one of said metallic grids toward and uniformly over the storage electrode.

' 3; "In a cathode-ray type storage tube having a storage electrode disposed at one end of an evacuated envelope, and'awriting'bearn gun adapted to scan the storage electrode: an annular cathode disposed transversely to and symmetrically about the longitudinal. axis of the envelope ands'paced'from the storage electrode; a focusing electrodev disposed partially about said cathode; and" two metallic grids disposed one on each side of said cathode to cause the electrons emitted at said cathode to form a virtual flood cathode in the space between said metallic grids, said emitted electrons being adapted to be directed toward the storage electrode.

4. A. cathode-ray type storage tube having a storage electrode for storing an electrostatic charge pattern, a writing beam gun, 'a virtual flood electron source for producing an electron stream to flood. the storage electrodewith electrons, and' an attracting electrode disposedbetween the electron source and' the storage electrode, said electron source comprising: an annular cathode sleeve; a filament di'sposed'within said cathode sleeve for heating said cathode; a toroidal focusing electrode having two rims extending radially inwardand partially enclosing said cathode sleeve;.t-wo metallic grids disposed-one on each side of' said cathode; and means for maintaining said cathode sleeve, said focusing electrode and'saictmetallicgrids at the same potential.

5., A cathode-ray type storage tube comprising: an

evacuated envelope; a writing-beam electron gun disposed at-oneend of said envelope for producing a writing beam of electrons; a storage electrode disposed at the opposite end of the envelope: for storing electrostatic charge patterns produced by the electrons of said writing beam gun; a deed beam electron. source including-an annular cathode a'focusing electrodedisposed aboutsaid cathode and two metallic grids disposed one on each side of said cathode, said electron source being disposed between said writing beam gun and said storage electrode; and means for directing said writing beam through said metallic grids tosaid storage; electrode.

6. A cathode-ray" type storage tube comprising: a

' storage-electrode; a floodbeam electron source compris- 57, 59; Electrons are then drawn through the interstices ing, an annular cathode; a, hollow toroidal focusing electrode having an; internal arcuate aperture disposed about said cathode; two metallic grids. disposed one on each side of said cathode; means for maintaining said focusing electrode at' a potential negative with respect to thev potential of'said cathode to focus the electrons "emitted from said cathode; and means for maintaining said metallic grid's at a potential positive with respect to that of said cathode to direct electrons emitted from said cathode radially inward into the space between said metallic grids.

7. A cathode-ray type storage tube comprising: an evacuated envelope; a writing beam electron gun disposed at one end of said envelope for producing a writing beam of electrons; a storage electrode disposed at the opposite end of the envelope for storing electrostatic charge patterns; and a flood beam electron source disposed between said writing beam gun and said storage electrode and including an annular cathode, a focusing electrode disposed about said cathode, means for maintaining said focusing electrode at a potential negative with respect to the potential of said cathode to focus the electrons emitted from said cathode, two metallic grids disposed one on each side of said cathode, and means for maintaining said metallic grids at a potential positive with respect to that of said cathode to direct electrons emitted from said cathode radially inward into the space between said metallic grids.

References Cited in the file of this patent UNITED STATES PATENTS Ring Jan. 9, 1940 Thompson Apr. 2, 1940 Epstein Mar. 30, 1943 Fox Oct. 26, Peter Oct. 4, Smith Dec. 27, Theile Mar. 13, Beintema Davis 

